This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 2001-338022 filed in JAPAN on Nov. 2, 2001, which is herein incorporated by reference.
(1) Field of the Invention
The present invention relates to a control method of causing an ink-jet head to eject ink by imparting energy to each of multiple ink chambers arranged adjoining the ink-jet head in accordance with image data as well as relating to an ink-jet printer for printing images using this control method.
(2) Description of the Prior Art
An ink-jet printer is a printer which prints images on recording media such as paper etc., by ejecting ink selectively from multiple ink chambers arranged adjoining an ink-jet head in accordance with image data, and is typically constructed such that, while a carriage having an ink-jet head mounted thereon is moved in the main scan direction perpendicular to the direction of conveyance of recording media, energy for causing ink to eject is applied to each of the ink chambers in accordance with image data. Such ink-jet heads can be categorized into two types, i.e., the thermal type which ejects ink by heating ink charged in ink chambers and the piezoelectric type which ejects ink by changing the volumes of ink chambers that hold ink therein.
The characteristics of a liquid ink used for image printing in ink-jet printers, such as viscosity and the like, are known to affect the ejection performance of ink from the ink chambers, having significant influence on the image forming conditions on the recording media and presenting sharp fluctuations depending on change in temperature. Therefore, to keep good print conditions of images on the recording sheet, temperature control of the ink-jet head is important.
Particularly, in thermal type ink-jet printers, since electric energy is imparted to each ink chamber of the ink-jet head and converted into thermal energy so as to heat the ink charged in the ink chamber, the ink ejection performance is liable to vary due to temperature rise of the whole ink-jet head. In addition to this, among the multiple ink chambers, some may be imparted with electric energy to eject ink, others may be imparted with no electric energy so as not to eject ink, resultantly a large difference in temperature occurs and hence produces fluctuations in ink ejection performance between the ejecting ink chambers and the non-ejecting ink chambers, lowering the image quality of printed images.
On the contrary, in piezoelectric type ink-jet printers in which piezoelectric elements are used to convert electric energy into mechanical energy so as to change the volumes of ink chambers, problems due to heat generation upon ink ejections, inherently, occur less often. However, among piezoelectric type ink-jet printers, there is a type that implements a so-called multi-drop printing process in which the tone of each pixel in the image is reproduced by up to seven serial ejections of ink as a maximum, for example, or with seven droplets of ink. With this type of ink-jet printer, as the frequency of electric energy applied to the ejecting ink chambers increases, generation of heat in the piezoelectric elements due to their deformation increases, hence causing the same problem as the thermal type ink-jet printers suffer, that is, temperature rise of the whole ink-jet head and increase in temperature difference between the ejecting ink chambers and the non-ejecting ink chambers, hence causing degradation of the image quality of printed images.
As a conventional ink-jet printer to deal with the above problems, Japanese Patent Application Laid-open Hei 3 No.246049 discloses a thermal type ink-jet printer configuration in which a certain amount of energy which will not cause ink ejection is applied to each of the non-ejecting ink chambers at the same time ink is ejected from ejecting ink chambers, so as to reduce the difference in ink temperature between the ejecting ink chambers and the non-ejecting ink chambers, keeping ink ejection performance uniform and preventing degradation of the image quality of printed images.
As another conventional example, Japanese Patent Application Disclosure Hei 11-511410 discloses a piezoelectric type ink-jet printer configuration in which drive pulses for heating are applied to each of non-ejecting ink chambers at the same time ink is ejected from ejecting ink chambers, so as to equalize the amount of heat generation from each ejecting ink chamber with that from each non-ejecting ink chamber, thereby keeping ink ejection performance uniform and preventing degradation of the image quality of printed images.
However, none of the conventional ink-jet printers including those disclosed in Japanese Patent Application Laid-open Hei 3 No.246049 and those disclosed in Japanese Patent Application Disclosure Hei 11-511410 have been manipulated so that when ink is ejected from the ink head, a specific amount of energy that can cause a temperature rise of the ink in the non-ejecting ink chambers equal to that of ink in the ejecting ink chambers can be imparted to each non-ejecting ink chamber. Therefore, in the conventional ink-jet heads, though energy is applied to each non-ejecting ink chamber at the same time ink is ejected from ejecting ink chambers, the temperatures of ink in all the ink chambers do not necessarily become equal, one to another, hence there still remains the problem of failure in reliably preventing the degradation of the image quality of printed images by uniformizing the ink ejection performance of all the ink chambers.
In sum, the ink in the ejecting ink chamber rises in temperature upon ejection of ink as it is heated by the difference between the quantity of heat generated by the input of energy for ejection and the quantity of heat carried away when the droplets of ink are ejected from the ejecting ink chamber. Accordingly, in order to cause ink in non-ejecting ink chambers to increase in temperature upon ejection of ink as much as the ink in the ejecting ink chambers and in order to make the ink in all the ink chambers arranged in the ink head substantially uniform in temperature, energy equivalent to the difference between the input of energy imparted to the ejection chamber and the quantity of energy carried away by the ink droplet should be imparted to each of the non-ejecting ink chambers.
It is therefore an object of the present invention to provide a control method of an ink-jet head and an ink-jet printer with the ink-jet head, wherein, upon ejection of ink, an amount of energy, the difference obtained by subtracting the energy carried away by ejected ink droplets that are ejected to the outside, from the energy imparted to each ejecting ink chamber, can be imparted to each of the non-ejecting ink chambers, so that the temperature of ink charged in the ejecting ink chambers and the temperature of ink charged in the non-ejecting ink chambers will be equal, and, upon ejection of ink, the ink in non-ejecting ink chambers is elevated in temperature as much as the increase in temperature of the ink in ejecting ink chambers, whereby it is possible to make the ink ejection performance as to all ink chambers provided for the ink-jet head substantially uniform and positively prevent degradation of the image quality of printed images.
In order to achieve the above described object, the present invention is configured as follows.
In accordance with the first aspect of the present invention, a method of controlling an ink-jet head having a multiple number of ink chambers arranged adjacent thereto for forming images by selectively imparting energy to each of the ink chambers in accordance with image data so as to cause ink charged in the ink chambers to eject, is characterized in that an amount of energy U0, which is determined by
U0=Uixe2x88x92Ud, 
is imparted to each of non-ejecting ink chambers for one ink ejection cycle, where Ui is the energy to be imparted to each ejecting ink chamber that ejects ink, every ink ejection cycle, among the multiple ink chambers, and Ud is the energy that is carried away by a single droplet of ink that is ejected to the outside when all the nozzles are driven to eject ink at the maximum ejection ratio with the temperature rise of the ink-jet head saturated.
In this configuration, upon ejection of ink from ejecting ink chambers to print an image, an amount of energy U0, the difference obtained by subtracting energy Ud carried away by one ejected ink droplet from energy Ui imparted to each ejecting ink chamber, is imparted to each of the non-ejecting ink chambers. Accordingly, the energy U0 equal to the energy (Uixe2x88x92Ud) consumed to heat ink in each ejecting ink chamber is imparted to each non-ejecting ink chamber when an action of ejection is made, so that ink inside the non-ejection chambers can be elevated in temperature as much as the increase in temperature inside the ejecting ink chambers, whereby the ink ejection performance as to all ink chambers provided for the ink-jet head can be made uniform no matter whether ink is ejected or not upon actions of ink ejection.
Here, the kinetic energy, surface energy and the energy consumed due ink viscosity of the ink droplets ejected from the ejecting ink chambers are sufficiently small compared to the energy used for generation of heat in the ejecting ink chambers and hence can be neglected.
The method of controlling an ink-jet head in accordance with the second aspect of the present invention, is characterized in that the energy U0 can be determined as
U0≈WF/(1+Cxc2x7xcex3xc2x7Vxc2x7Rt)/N, 
and is imparted to each non-ejecting ink chamber every time ink is ejected from the ejecting ink chambers,
where WF(W) is the input electric power when all ink chambers are caused to eject ink so that N ink droplets are ejected every second from the entire ink-jet head, C(J/(gxc2x7deg)) is the specific heat of the ink, xcex3(g/cc) is the specific weight of ink, V(cc/sec) is the amount of ejected ink and Rt(deg/W) is the heat resistance of the ink-jet head including radiator parts.
In this configuration, when a volume V(cc/sec) of ink having a specific heat of C(J/(gxc2x7deg)) and a specific weight of xcex3(g/cc) is ejected from all ink chambers provided for an ink-jet head presenting a heat resistance Rt(deg/W) as a self-heat releasing performance to the outside air, N droplets of ink are ejected every second from the whole ink-jet head (N is the product of the total number n of ink chambers in the ink-jet head and the ejection frequency f of ink droplets). In this case, an amount of energy U0, which is determined by
U0≈WF/(1+Cxc2x7xcex3xc2x7Vxc2x7Rt)/N, 
where WF(W) is the input electric power,
is imparted to each non-ejecting ink chamber every time one ink ejection cycle is made. Accordingly, the energy to be imparted to each non-ejecting ink chamber upon an action of ink ejection can be optimized in terms of heat balance, based on the power consumption and the total number of ink droplets ejected for one second when all the ink chambers provided for the ink-jet head are caused to eject ink. As a result, the ink ejection performance in all ink chambers provided for the ink-jet head, can be kept substantially uniform no matter whether ink is ejected or not, when ink is ejected.
The method of controlling an ink-jet head according to the third aspect of the present invention is characterized in that the ink-jet head comprises a thermal type ink-jet head which ejects ink by converting the electric energy input to each ink chamber into thermal energy.
In the configuration which uses a thermal type ink-jet head, though a large temperature difference is liable to arise between the ejecting ink chambers and the non-ejecting ink chambers since ink is ejected by imparting electric energy to each ink chamber of the ink-jet head and converting it into thermal energy so as to heat the ink charged in the ink chamber, an amount of heat energy equal to the heat energy used for heating ink in the ejecting ink chamber upon an action of ink ejection, is imparted to each non-ejecting ink chamber. Accordingly, it is possible to increase the temperature of the ink in each non-ejecting ink chamber as much as the ink in ejecting ink chambers, whereby the ink ejection performance in all ink chambers provided for the ink-jet head, can be kept substantially uniform no matter whether ink is ejected or not, when ink is ejected.
The method of controlling an ink-jet head according to the fourth aspect of the present invention is characterized in that the ink-jet head comprises a piezoelectric type ink-jet head which ejects ink by converting the electric energy input to each ink chamber into mechanical energy.
In this configuration which uses a piezoelectric type ink-jet head, though heat is generated by deformation of piezoelectric elements since electric energy imparted to each ink chamber is converted into mechanical energy so as to change the volumes of the ink chambers by deformation of the piezoelectric elements, an amount of energy equal to the energy which will cause a temperature rise of the piezoelectric element in each ejection chamber upon an action of ink ejection, is imparted to each non-ejecting ink chamber. Accordingly, it is possible to cause the piezoelectric element in each non-ejecting ink chamber to generate as much heat as the piezoelectric element provided in each ejecting ink chamber does, hence it is possible to heat the ink in each non-ejecting ink chamber in an equivalent way to the way in which the ink in each ejecting ink chamber is heated, whereby the ink ejection performance in all ink chambers provided for the ink-jet head, can be kept substantially uniform no matter whether ink is ejected or not, when ink is ejected.
The method of controlling an ink-jet head according to the fifth aspect of the present invention is characterized in that drive energy is imparted to the ink chambers a number of times, up the specified maximum number, in accordance with image density data, during one cycle of a series of ink droplets.
In a so-called multi-drop type ink-jet head, a remarkable temperature difference in ink temperature between the ejecting ink chambers and the non-ejecting ink chambers upon ejection of ink is liable to occur because energy imparted to the ejection ink chambers is applied in a relatively high frequency in order for each pixel in the image to be reproduced by an ink droplet group, consisting of a single or multiple ink droplets, up to the predetermined maximum number, in accordance with image density data. In the configuration of the present invention, an amount of energy equal to the energy used for heating ink in the ejecting ink chamber upon an action of ink ejection, is imparted to each non-ejecting ink chamber. Accordingly, even with a multi-drop type ink-jet head, the difference in temperature between the ejecting ink chambers and the non-ejecting ink chambers upon actions of ink ejection will never become too much.
The sixth aspect of the present invention resides in an ink-jet printer comprising a controller, which controls an ink-jet head having a multiple number of ink chambers arranged adjacent thereto for forming images by selectively imparting energy to each of the ink chambers in accordance with image data so as to cause ink charged in the ink chambers to eject, and which implements a control method whereby an amount of energy U0, which is determined by
U0=Uixe2x88x92Ud, 
is imparted to each of non-ejecting ink chambers for one ink ejection cycle, where Ui is the energy to be imparted to each ejecting ink chamber that ejects ink, every ink ejection cycle, among the multiple ink chambers, and Ud is the energy that is carried away by a single droplet of ink that is ejected to the outside when all the nozzles are driven to eject ink at the maximum ejection ratio with the temperature rise of the ink-jet head saturated.
In this configuration, when, among the multiple ink chambers arranged adjoining an ink-jet head, energy is imparted to ejecting ink chambers selected in accordance with image data, an amount of energy U0, the difference obtained by subtracting energy Ud carried away by the ejected ink droplet from energy Ui imparted to each ejecting ink chamber, is imparted to each of the non-ejecting ink chambers other than the ejecting ink chambers. Accordingly, the energy U0 equal to the energy (Uixe2x88x92Ud) consumed to heat ink in each ejecting ink chamber is imparted to each non-ejecting ink chamber when an action of ejection is made, so that ink inside the non-ejection chambers can be elevated in temperature as much as the increase in temperature inside the ejecting ink chambers, whereby it is possible to make the ink ejection performance, as to all ink chambers provided for the ink-jet head, uniform, and hence keep good image forming conditions.